7,887 research outputs found
Diversity-Multiplexing Tradeoffs in MIMO Relay Channels
A multi-hop relay channel with multiple antenna terminals in a quasi-static
slow fading environment is considered. For both full-duplex and half-duplex
relays the fundamental diversity-multiplexing tradeoff (DMT) is analyzed. It is
shown that, while decode-and-forward (DF) relaying achieves the optimal DMT in
the full-duplex relay scenario, the dynamic decode-and-forward (DDF) protocol
is needed to achieve the optimal DMT if the relay is constrained to half-duplex
operation. For the latter case, static protocols are considered as well, and
the corresponding achievable DMT performance is characterized.Comment: To appear at IEEE Global Communications Conf. (Globecom), New
Orleans, LA, Nov. 200
Minimum Sparsity of Unobservable Power Network Attacks
Physical security of power networks under power injection attacks that alter
generation and loads is studied. The system operator employs Phasor Measurement
Units (PMUs) for detecting such attacks, while attackers devise attacks that
are unobservable by such PMU networks. It is shown that, given the PMU
locations, the solution to finding the sparsest unobservable attacks has a
simple form with probability one, namely, , where
is defined as the vulnerable vertex connectivity of an augmented
graph. The constructive proof allows one to find the entire set of the sparsest
unobservable attacks in polynomial time. Furthermore, a notion of the potential
impact of unobservable attacks is introduced. With optimized PMU deployment,
the sparsest unobservable attacks and their potential impact as functions of
the number of PMUs are evaluated numerically for the IEEE 30, 57, 118 and
300-bus systems and the Polish 2383, 2737 and 3012-bus systems. It is observed
that, as more PMUs are added, the maximum potential impact among all the
sparsest unobservable attacks drops quickly until it reaches the minimum
sparsity.Comment: submitted to IEEE Transactions on Automatic Contro
Distortion Exponent in MIMO Channels with Feedback
The transmission of a Gaussian source over a block-fading multiple antenna
channel in the presence of a feedback link is considered. The feedback link is
assumed to be an error and delay free link of capacity 1 bit per channel use.
Under the short-term power constraint, the optimal exponential behavior of the
end-to-end average distortion is characterized for all source-channel bandwidth
ratios. It is shown that the optimal transmission strategy is successive
refinement source coding followed by progressive transmission over the channel,
in which the channel block is allocated dynamically among the layers based on
the channel state using the feedback link as an instantaneous automatic repeat
request (ARQ) signal.Comment: Presented at the IEEE Information Theory Workshop (ITW), Taormina,
Italy, Oct. 200
The Multi-way Relay Channel
The multiuser communication channel, in which multiple users exchange
information with the help of a relay terminal, termed the multi-way relay
channel (mRC), is introduced. In this model, multiple interfering clusters of
users communicate simultaneously, where the users within the same cluster wish
to exchange messages among themselves. It is assumed that the users cannot
receive each other's signals directly, and hence the relay terminal in this
model is the enabler of communication. In particular, restricted encoders,
which ignore the received channel output and use only the corresponding
messages for generating the channel input, are considered. Achievable rate
regions and an outer bound are characterized for the Gaussian mRC, and their
comparison is presented in terms of exchange rates in a symmetric Gaussian
network scenario. It is shown that the compress-and-forward (CF) protocol
achieves exchange rates within a constant bit offset of the exchange capacity
independent of the power constraints of the terminals in the network. A finite
bit gap between the exchange rates achieved by the CF and the
amplify-and-forward (AF) protocols is also shown. The two special cases of the
mRC, the full data exchange model, in which every user wants to receive
messages of all other users, and the pairwise data exchange model which
consists of multiple two-way relay channels, are investigated in detail. In
particular for the pairwise data exchange model, in addition to the proposed
random coding based achievable schemes, a nested lattice coding based scheme is
also presented and is shown to achieve exchange rates within a constant bit gap
of the exchange capacity.Comment: Revised version of our submission to the Transactions on Information
Theor
Sosa, Customary International Law, and the Continuing Relevance of Erie
Ten years ago, the conventional wisdom among international law academics was that customary international law (CIL) had the status of self-executing federal common law to be applied by courts without any need for political branch authorization. This modern position came under attack by so-called revisionist critics who argued that CIL had the status of federal common law only in the relatively rare situations in which the Constitution or political branches authorized courts to treat it as such. Modern position proponents are now claiming that the Supreme Court\u27s 2004 decision in Sosa v. Alvarez-Machain confirms that CIL has the status of self-executing federal common law. As this Article explains, the decision in Sosa did not in fact embrace the modern position, and, indeed, is best read as rejecting it. Commentators who construe Sosa as embracing the modern position have confounded the automatic incorporation of CIL as domestic federal law in the absence of political branch authorization (i.e., the modern position) with the entirely different issue of whether and to what extent a particular statute, the Alien Tort Statute ( ATS ), authorizes courts to apply CIL as domestic federal law. The Article also explains how CIL continues to be relevant to domestic federal common law despite Sosa\u27s rejection of the modern position. The fundamental flaw of the modern position is that it ignores the justifications for, and limitations on, post-Erie federal common law. As the Article shows, however, there are a number of contexts in addition to the ATS in which it is appropriate for courts to develop federal common law by reference to CIL, including certain jurisdictional contexts not amenable to state regulation (namely admiralty and interstate disputes), and gap-filling and interpretation of foreign affairs statutes and treaties. The Article concludes by considering several areas of likely debate during the next decade concerning the domestic status of CIL: corporate aiding and abetting liability under the ATS; application of CIL to the war on terrorism; and the use of foreign and international materials in constitutional interpretation
Some potential blood flow experiments for space
Blood is a colloidal suspension of cells, predominantly erythrocytes, (red cells) in an aqueous solution called plasma. Because the red cells are more dense than the plasma, and because they tend to aggregate, erythrocyte sedimentation can be significant when the shear stresses in flowing blood are small. This behavior, coupled with equipment restrictions, has prevented certain definitive fluid mechanical studies from being performed with blood in ground-based experiments. Among such experiments, which could be satisfactorily performed in a microgravity environment, are the following: (1) studies of blood flow in small tubes, to obtain pressure-flow rate relationships, to determine if increased red cell aggregation can be an aid to blood circulation, and to determine vessel entrance lengths, and (2) studies of blood flow through vessel junctions (bifurcations), to obtain information on cell distribution in downstream vessels of (arterial) bifurcations, and to test flow models of stratified convergent blood flows downstream from (venous) bifurcations
Joint Source-Channel Cooperative Transmission over Relay-Broadcast Networks
Reliable transmission of a discrete memoryless source over a multiple-relay
relay-broadcast network is considered. Motivated by sensor network
applications, it is assumed that the relays and the destinations all have
access to side information correlated with the underlying source signal. Joint
source-channel cooperative transmission is studied in which the relays help the
transmission of the source signal to the destinations by using both their
overheard signals, as in the classical channel cooperation scenario, as well as
the available correlated side information. Decode-and-forward (DF) based
cooperative transmission is considered in a network of multiple relay terminals
and two different achievability schemes are proposed: i) a regular encoding and
sliding-window decoding scheme without explicit source binning at the encoder,
and ii) a semi-regular encoding and backward decoding scheme with binning based
on the side information statistics. It is shown that both of these schemes lead
to the same source-channel code rate, which is shown to be the "source-channel
capacity" in the case of i) a physically degraded relay network in which the
side information signals are also degraded in the same order as the channel;
and ii) a relay-broadcast network in which all the terminals want to
reconstruct the source reliably, while at most one of them can act as a relay.Comment: Submitted to IEEE Transactions on Information Theory, 201
Test evaluation of fuel cell catalysts Final report
Carbides, bromides,nitrides,and alloys tested for catalytic characteristics for ammonia and carbon nonoxide in fuel cell electrolyte
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